[LLDB][MIPS] Emulate instruction using MCDisassembler

Patch by Jaydeep Patil

EmulateInstructionMIPS64 has been modified to use llvm::MCDisassembler instead of duplicating the decoding logic.
Added emulation of few branch instructions for software single stepping

Reviewers: clayborg, jasonmolenda
Subscribers: bhushan, mohit.bhakkad, sagar, lldb-commits.
Differential Revision: http://reviews.llvm.org/D9319

llvm-svn: 236411